Mixed reaction.
I've never personally played a character I didn't create, but the first Third Edition game I played had two characters in it that originated as player-played "NPCs" who stuck around after the session they were created for.
Backstory: My character and another PC were magically imprisoned within a forest shack with a half-dozen NPCs by a slightly-disturbed cleric. Long story short, the DM asked two friends of ours to play two of the important NPCs - Isobel and Corvus - for that session, and they, having had a good time, decided to stick around and keep playing those characters.
Their experiences were different. My best friend, an inexperienced roleplayer at the time, played Isobel, and ended up being somewhat sorry that she hadn't simply created her own character, since the choices the DM had made so Isobel would fit the session he'd written her for ended up being somewhat detrimental to the way my friend ended up playing the character.
On the other hand, the other player, much more experienced and knowing that Corvus was to be short-lived (since he joined during a period where the original campaign was split into two groups, and he already had a PC in the other half of the campaign) played him to the hilt, and wasn't even all that sorry when the party murdered him shortly before meeting up with the other half of the campaign.
(Yeah, it was an evil/neutral party. Isobel was a CE werefox cleric of Cyric (and it's the werefox part that ended up getting in the way), and Corvus was a CE fighter rogue who once donned a helm of opposite alignment and declared himself a paladin of Cyric in front of an NPC merchant caravan. Heh.)
Personally, I feel that the DM has the right to veto character concepts that don't fit her world, the rest of the party, or her campaign, but not to pressure players into playing a character which conforms to her vision. As I said, I've never played a DM-created character, but the campaign I mention above was eventually ruined for me when the DM started imposing his own narrative ideas of where the characters (as opposed to the background story of the game) should be going without regard for the players' own ideas about their characters' nature, goals, and choices - which I think is a very similar situation.